There have been recent developments in the miniaturation of various electro-mechanical devices also known as micro machines. From this push to miniaturize, the field of diffraction gratings or now commonly referred to as grating light valves has emerged. An example of a GLV is disclosed in U.S. Pat. No. 5,311,360 which is incorporated in its entirety herein by reference. According to the teachings of the '360 patent, a diffraction grating is formed of a multiple mirrored-ribbon structure such as shown in FIG. 1. A pattern of a plurality of deformable ribbon structures 100 are formed in a spaced relationship over a substrate 102. Both the ribbons and the substrate between the ribbons are coated with a light reflective material 104 such as an aluminum film. The height difference that is designed between the surface of the reflective material 104 on the ribbons 100 and those on the substrate 102 is .lambda./2 when the ribbons are in a relaxed, up state. If light at a wavelength .lambda. impinges on this structure perpendicularly to the surface of the substrate 102, the reflected light from the surface of the ribbons 100 will be in phase with the reflected light from the substrate 102. This is because the light which strikes the substrate travels .lambda./2 further than the light striking the ribbons and then returns .lambda./2, for a total of one complete wavelength .lambda.. Thus, the structure appears as a flat mirror when a beam of light having a wavelength of .lambda. impinges thereon.
By applying appropriate voltages to the ribbons 100 and the substrate 102, the ribbons 100 can be made to bend toward and contact the substrate 102 as shown in FIG. 2. The thickness of the ribbons is designed to be .lambda./4. If light at a wavelength .lambda. impinges on this structure perpendicularly to the surface of the substrate 102, the reflected light from the surface of the ribbons 100 will be completely out of phase with the reflected light from the substrate 102. This will cause interference between the light from the ribbons and light from the substrate and thus, the structure will diffract the light. Because of the diffraction, the reflected light will come from the surface of the structure at an angle .THETA. from perpendicular.
In formulating a display device, one very important criteria is the contrast ratio between a dark pixel and a lighted pixel. The best way to provide a relatively large contrast ratio is to ensure that a dark pixel has no light. One technique for forming a display device using the structure described above, is to have a source of light configured to provide light with a wavelength .lambda. which impinges the surface of the structure from the perpendicular. A light collection device, e.g., optical lenses, can be positioned to collect light at the angle .THETA.. If the ribbons for one pixel are in the up position, all the light will be reflected back to the source and the collection device will receive none of the light. That pixel will appear black. If the ribbons for the pixel are in the down position, the light will be diffracted to the collection device and the pixel will appear bright.
Experimentation has shown that the turn-on and turn-off voltages for GLV ribbons exhibit hysteresis. FIG. 3 shows a brightness versus voltage graph for the GLV. The vertical axis represents brightness and the horizontal axis represent voltage. It will be understood by those of ordinary skill in the art that if diffracted light is collected, when the GLV ribbon is up and at rest, that the minimum of light is collected. When the GLV ribbon is down, the maximum of light is collected. In the case where the ribbon is able to move downwardly by exactly .lambda./4 of the wavelength of the anticipated light source, then the light collected in the down position with the ribbon firmly against the substrate is truly at a maximum.
Upon initial use, the GLV remains in a substantially up position while at rest, thereby diffracting no light. To operate the GLV, a voltage is applied across the ribbon 100 (FIG. 1) and the underlying substrate 102. As the voltage is increased, almost no change is evident until a switching voltage V.sub.2 is reached. Upon reaching the switching voltage V.sub.2, the ribbon snaps fully down into contact with the substrate. Further increasing the voltage will have negligible effect on the optical characteristics of the GLV as the ribbon 100 is fully down against the substrate 102. Though the ribbon is under tension as a result of being in the down position, as the voltage is reduced the ribbon does not lift off the substrate until a voltage V.sub.1 is reached. The voltage V.sub.1 is lower than the voltage V.sub.2. This initial idealized operating characteristic is shown by the solid line curve 106 in FIG. 3.
The inventors discovered that the GLV devices exhibited aging. It was learned that operating the GLV over an extended period of time caused the release voltage to rise toward the switching voltage V.sub.2. Additionally, the amount of diffracted light available for collection also decreased as the release voltage increased. Experience led the inventors to realize that the GLV devices were fully aged after about one hour of continuously switching the GLV between the up and relaxed state to the down and tensioned state. These experiments were run at 10,000 Hz. Though those previous inventions worked as intended, this change in release voltage and the degradation of the diffracted light made such GLV devices unsuitable as commercial production products.
FIG. 4 shows an actual graph for the amount of light versus voltage for a control GLV device operated in an ambient atmosphere. A series of five curve traces are shown, 108, 110, 112, 114 and 116. Each of the traces is taken at a different point during the aging cycle, trace 108 being recorded first in time, and then each successive trace recorded at a later point in the aging cycle. FIG. 4 shows the voltage applied both positively and negatively. What the traces of FIG. 4 show is that after the ribbon 100 (FIG. 1) is forced into the down position against the substrate 102 at a voltage V.sub.2, reducing the applied voltage will cause the amount of the collected diffracted light to diminish until the release voltage V.sub.1 is reached. This phenomenon is likely reached as the edges of the ribbon 100 begin to rise. However, as long as at least a portion of the ribbon 100 remains in contact with the substrate 102, a significant portion of the light is diffracted and hence available for collection.
It is apparent from FIG. 4 that each recorded successive trace 110, 112, 114 and 116 shows that the release voltage V.sub.1 continues to rise and concurrently the amount of collected diffracted light decreases. FIG. 5 is a corresponding graph to FIG. 4 and shows the switching voltage V.sub.2 and the release voltage V.sub.1 during the aging process. The voltage levels are shown on the vertical axis and time is shown in the horizontal axis. FIG. 5 shows that the switching voltage V.sub.2 remains fairly stable during the aging process. However, FIG. 5 also shows that the release voltage V.sub.1 rises during the aging cycle.
Analysis of GLVs after the completion of the aging cycle shows that structures build between the ribbon surface and the underlying substrate. FIG. 6 schematically shows that structures can develop on the bottom of a ribbon 120 while the substrate 122 remains relatively unchanged. FIG. 7 schematically shows that structures can develop on the top of the substrate 124 while the bottom of a ribbon 126 remains relatively unchanged. FIG. 8 schematically shows that structures can develop on the bottom of a ribbon 128 and also on the top of the substrate 130. As the irregularities 132 develop, the ribbons 120, 126 and 128 are prevented from moving all the way down onto the substrate 122, 124 and 130, respectively. The irregularities prevent the ribbons from moving .lambda./4 of the anticipated wavelength of incident light. Hence, incomplete diffraction into collection optics results and the maximum light level achievable is reduced.
It is believed that the irregularities grow as a result of the contact between the GLV ribbon and the substrate. The ribbon impacts the substrate at relatively high rate of speed. Upon contact of the ribbon onto the substrate, the surfaces join together in a welding-like process. As the surfaces release from one another, a portion of one of the surfaces releases forming a raised irregularity on the surface to which the welded structure remains adhering. Over time this process continues until the irregularity negatively impacts the operation of the structure.
As shown in FIG. 9, in operation, the GLV ribbon preferably is toggled into the down state by increasing the voltage above the switching voltage V.sub.s. Then the voltage is lowered to and maintained at a biasing voltage V.sub.B. To raise the GLV ribbon to the up state, the voltage is lowered below the release voltage V.sub.R. The voltage is then raised and maintained at the biasing voltage V.sub.B. In this way no change in optical characteristics occurs by changing the voltage to the biasing voltage V.sub.B, yet the amount of voltage necessary to change the state of the GLV ribbon is a small pulse in either direction. Unfortunately, as the release voltage changes, such operation can become unstable.
The assignee of this application has developed another GLV technology called the flat GLV. That technology is disclosed in U.S. patent application Ser. No. 08/482,188, filed Jun. 7, 1995, entitled Flat Diffraction Grating Light Valve and invented by David M. Bloom, Dave B. Corbin, William C. Banyai and Bryan P. Staker. This application is allowed and will issue on Nov. 24, 1998 as U.S. Pat. No. 5,841,579. This patent document is incorporated herein by reference. All the same problems associated with aging also apply to the flat GLV technology.
What is needed is a solution that prevents the surfaces of two elements which contact each other in a GLV from adhering or sticking to each other and thereby prevent the formation of irregularities therebetween. Additionally, a method is needed for carrying out the solution in a manufacturing process of the GLV.